Natural rubber latex film products with improved tear resistance

ABSTRACT

Natural rubber latex compounds are modified by the addition of a high styrene content styrene-butadiene copolymer. The films prepared from these modified NR latex compositions exhibit improved tear strength and a good balance of other properties.

The invention relates to natural rubber latex film products whichcontain a high styrene content styrene-butadiene latex additive. Thelatex rubber film products of the invention exhibit improved tearresistance.

BACKGROUND OF THE INVENTION

Many prophylactic health care products such as condoms, diaphragms,medical and surgeons' gloves are made from natural rubber ("NR"). NR isavailable in pure latex form as recovered from the Hevea Brasiliensistree. Thus the preferred manufacturing process is based on the directconversion of the NR latex into the finished product. This is readilyaccomplished by dipping shaped-formers into compounded natural rubberlatex and removing the water by drying in an oven. Natural Rubber, whichis a pure form of stereoregular cis-1,4-polyisoprene, has a very highmolecular weight in its native form, and therefore readily forms acontinuous film in the dipping/drying process. Also, in order tostabilize the polymeric microstructure, the NR latex is compounded withcuratives which cross-link the individual polymer molecules whenadditional heat is applied.

The cross-linked natural rubber film exhibits a relatively low modulus,giving it good flexibility and extensibility. However, the film alsoexhibits exceptionally high tensile strength due to straincrystallization reinforcement and a very high level of extension attensile failure.

The major deficiency of natural rubber is its relatively rapid reversionand degradation on aging, which is initiated by heat, oxygen, ozone, andbiological agents. Also as NR films age, they exhibit a tendency topremature failure due to a lowered level of tear resistance.

In the past, there has not been any great urgency to address theimprovement in physical properties of natural rubber latex films.However, with the increasing incidence of serious sexually-transmitteddiseases, there is significant concern about the integrity andreliability of prophylactic products. The specific objective of thestudy that led to this invention was to improve the initial and retainedtear strength of natural rubber films, which can then be used to produceimproved prophylactic condoms, medical and surgical gloves, finger cots,and similar medical products. The invention is also applicable to otherproducts that are made from NR latex films, such as balloons.

SUMMARY OF THE INVENTION

Natural rubber latex compounds are modified by the addition of a highstyrene content styrene-butadiene copolymer. The films prepared fromthese modified NR latex compositions exhibit improved tear strength anda good balance of other properties.

THE PRIOR ART

Published UK patent application No. GB 2 088 389 A, published on June 9,1982, discloses the use of polyvinyl chloride as an additive to NR latexto improve the tear and puncture resistance of articles made from thelatex.

Natural rubber has been blended with synthetic rubbers, includingstyrene-butadiene copolymer elastomers ("SBR's"), usually in the bulkphase, since the 1930's. The commodity SBR elastomers are based oncompositions with about 25% bound styrene content, the remainder of thecopolymer being butadiene. Such compositions exhibit good rubberycharacteristics with moderate strength. Thus, blends of natural rubberand SBR give useful compositions at low cost which are used in themanufacture of tires and mechanical goods.

Illustrative disclosures of rubber articles made from mixtures ofnatural rubber and styrene-butadiene copolymers include Hashimoto etal., U.S. Pat. No. 4,590,123 (see, especially, Table 6 in Col. 7), andVazquez, U.S. Pat. No. 4,356,824 (Col. 11, lines 6-8).

Mochizuki et al., in U.S. Pat. No. 4,675,347, disclose antimicrobiallatex compositions containing NR latex and an antimicrobial agent. Amongthe NR latex compositions disclosed is one containing NR latex and astyrene-butadiene copolymer latex (Col. 10, lines 60 et seq.)

Kavalir et al., in U.S. Pat. No. 3,286,011, disclose rubber gloves thathave a non-slip coating on the surface. The non-slip coating may be amixture of NR latex and a high styrene content styrene-butadienecopolymer latex.

Teague, in U.S. Pat. No. 2,747,229, discloses fabric-lined rubbergloves. The rubber may be a mixture of a styrene-butadiene copolymer andmilled raw rubber (col. 5, lines 11-19).

DETAILED DESCRIPTION OF THE INVENTION

The styrene-butadiene latexes that are blended with natural rubber inaccordance with the invention are the high styrene contentstyrene-butadiene ("SB") latexes. Such latexes have greater than 50weight percent, up to, for example, about 90 weight per cent, andpreferably from about 75 to about 85 weight percent, styrene in thecopolymer comprising the (SB) latex, the remainder of the copolymerbeing butadiene.

The addition of the SB latex to the NR latex, in accordance with theinvention, does not require any significant modification of thecustomary production procedures that are followed when making NRarticles by the latex dipping process. The two latexes (i.e., the SB andthe NR latex) are simply mixed in the appropriate proportions, alongwith the other additives that are customarily employed in the latexdipping process, and the latex is then used in the customary manner. TheSB latex is used in proportions such that the tear strength of the NRlatex is improved, without having any adverse effect on otherproperties. As a general rule, the SB latex will be used in proportionsof from about 1 part to about 25 parts, and preferably from about 10 toabout 20 parts, by weight, per hundred parts by weight of the NR latex.(These proportions are on a solids basis.)

The invention is useful in the production of prophylactic condoms,medical and surgical gloves, finger cots, prophylactic diaphrams,balloons, and other articles that are made from NR latex films.

The experimental section, below, illustrates the practice of theinvention.

EXPERIMENTAL PROCEDURES

Films from blends of natural rubber and an SB copolymer latex wereprepared in two separate experimental studies in order to confirm therepeatability of the findings. In each experimental study, a masterbatchcompound of natural rubber latex was prepared as a control and as a basefor blending with SB copolymer latex.

The film dipping was carried out with a 5 station laboratory dippingmachine. The aluminum dipping formers are cylindrical, with a diameterof 5 cm.

The formers were held stationary, and the vessels containing the latexwere raised so that the formers were immersed in latex. At the beginningof the dipping sequence, the vessels containing the various latexmixtures are located on a tray mounted directly beneath the formers. Thetray is hydraulically lifted at a speed of 40 cm/min, so that theformers are almost completely immersed into the latex.

In the second step, the tray is lowered at a speed of 20 cm/min untilthe formers are completely out of the latex.

Immediately thereafter, the formers are rotated around theirlongitudinal axes at a speed of 10 rpm, and they are moved into ahorizontal position. While rotating, they are irradiated with a Boekamp1500 W Quartz heater (Model 1001), at a distance of 20 cm from theformers. Then the dipping and drying sequence is repeated to give adouble layer of continuous rubber film. It is standard practice to usethis double-dipping procedure in the production of condoms in order toinsure the absence of pinholes. When using the invention to produceother NR latex film products, known modifications to the process can beused. For instance, in producing surgical and medical gloves, normallyonly one latex dip is used, which dip is preceded by a dip in acoagulant (a multi-valent metal salt such as calcium nitrate, calciumchloride, zinc chloride, in a suitable solvent, along with surfactants).The coagulant serves to coalesce the latex particles in a uniformmanner. After the latex dip, when a coagulant is used, a leaching stepis normally used to remove the coagulant salts prior to thedrying/curing step.

The formers are removed from the dipping machine, and placed into acuring oven for 35 minutes at 100° C. After curing, the formers aretaken out and cooled for 15 minutes. The film is then dusted with talc,and removed by a simple pulling action.

The physical property data for the NR films are used as controls incomparative testing of the properties of films made from the blends.

A blend of natural rubber latex and SB latex on an 80/20 w/w solidsbasis was also prepared. This blend and the NR latex control batch wereadmixed in proportion to give blends with 5, 10, 15, and 20 phr (i.e.,parts per hundred parts of NR latex) of styrene-butadiene copolymercomponent in natural rubber.

Films were prepared by the double-dipping procedure described above.Separate hollow cylindrical formers were dipped simultaneously intoseparate vessels containing the control and the 5, 10, 15, and 20 phrSB-NR blends. The individual films were dried and then vulcanized in anair convection oven at 100° C. for 35 minutes.

The tear strength and tensile strength on flat films were determinedusing an Instron 4201 instrument and the procedures of ASTM D624 andD412 respectively. The tensile strength of cylindrical hoop sectionswere also measured using a rotating spindle holder according to theprocedure of ASTM D3492 and D412.

Also some film samples were aged in an air convection oven at 70° C. forone week, and then tested for tear strength.

The films were stripped from the formers after curing, using talc tominimize tack and self-adhesion. Test samples were cut from the rubberfilms using steel dies and a Carver press.

Physical testing was conducted at room temperature with a minimum ofaging time.

TEAR STRENGTH TEST

Tear strength was evaluated using the Instron series IX automatedmaterials testing system, and the ASTM method D624-54. The specimenswere cut using die C (90° notch) and the thickness (T) of each specimenat the notch was measured using a micrometer. The gauge length (GL) was70 mm and the crosshead speed of the Instron was 500 mm/min The tearstrength, in N/cm, was calculated from the equation F/T, wherein themaximum load (F) was measured by the Instron. The break elongation wascalculated by the equation (D/GL)×100, wherein the displacement at break(D) was measured by the Instron system.

TENSILE STRENGTH TEST USING DUMBELL SHAPED DIE

According to the ASTM method D412-83, the tensile strength of specimenscut from dumbell die C was obtained using an Instron series IX automatedmaterials testing system. The gauge length (GL) used was 50 mm and thecrosshead speed of the Instron was 500 mm/min. The thickness of eachspecimen, taken from the centre, was measured using a micrometer. Thetensile strength, in MPa, was calculated from the equation F/A, whereinthe load (F) was measured by the Instron and the area (A) was calculatedfrom the width of die C and the individual thicknesses of the specimens.The percent strain at break was calculated by the equation (D/GL)×100,wherein the displacement at break (D) is measured by the Instron system.

TENSILE STRENGTH USING RING SPECIMENS

The tensile strength was obtained using the ASTM method D3492-83 and anInstron series IX automated materials testing system. The gauge lengthor the distance between the centres of the rollers was 30 mm and thecrosshead speed of the Instron was 500 mm/min. The minimum thickness (T)of each specimen was measured using a micrometer. The tensile strength,in MPa, was calculated using the equation F/(2WT), wherein the load atbreak (F) was measured by the Instron and the width (W) of the ring orthe die width was 20 mm. The percent strain at break was calculated fromthe equation 100×(2D/C) wherein the displacement at break (D) ismeasured by the Instron system and C is the circumference of the ringspecimen.

EXAMPLE 1

The individual NR latex masterbatch and 20 phr blend were prepared inopen mixing vessels of about 10 liters volume capacity each. Theformulations of the dry ingredients and of the dispersions are given inTable 1 and Table 2, respectively.

In the preparation of the above batches, the ingredients are combined inthe order of listing in the tables with constant agitation. The blendsof intermediate composition were prepared by admixing the NR latexmasterbatch and 20 phr SBR blend in the appropriate proportions, e.g.,468 g of the 20% SB blend diluted with 1404 g of NR latex compound gavethe 5% blend.

The blends were transferred to sealable jars and agitated on a rollermixing machine for 48 hours prior to dipping. Films were prepared bydipping hollow cylindrical formers directly into the jars using aspecial machine built for this purpose, as described above. Entry andwithdrawal rates of the formers were closely controlled to give films ofuniform thickness. While still mounted on the machine, the films weredried using an infrared lamp and a portable hot air dryer. The formerscovered with rubber film were removed from the machine and placed in anair oven at 100° C. for 35 minutes.

The cured films from this experiment were cut into test specimens usingthe ASTM Die C for tear test specimens and a dumbell die for thespecimens for tensile strength measurements. A summary of tear strengthand tensile strength data is given in Table 3 and Table 4, respectively.

EXAMPLE 2

The experimental study of Example 1 was repeated using a more recentdelivery of Natural Rubber latex and freshly compounded curativedispersions. More than 60 test specimens were prepared and tested inorder to conduct a statistical analysis (refer to Tables 6 and 7).

DISCUSSION OF TEST RESULTS Tear Strength

The films of Example 1 and Example 2 were tested separately for theirtear strength, as a measure of tear resistance, using the ASTM procedureD-624-54 and samples cut with Die C. In Example 1, about 25 testspecimens were prepared, whereas in Example 2, approximately 60 testspecimens were tested.

The data in Table 6 shows that the natural rubber film has a tearstrength of about 630 N/cm average. However, there is a considerablerange of values indicating that there could be a wide variability inindividual film thickness and distribution of surface flaws. There is aconsiderable variation in average film thickness. This can be attributedin part to surface tension and viscosity effects, particularly inrelation to the improvement observed for the blends with the syntheticlatex.

When 5 phr of the styrene-butadiene copolymer is added to naturalrubber, the latex films exhibit about a 50% increase in tear strength.Similarly, the addition of 10% and 15% SB copolymer provides a furtherincrease in tear strength, with some values double the values for thenatural rubber film alone.

It is noted that the consistency of the test results seem to improvewith the addition of the synthetic latex. This is likely attributable inpart to the presence of additional emulsifiers as well as the physicalreinforcement phenomenon. The formulation used for control dipping isbasic. In commercial practice, other additives would be included tooptimize surface tension and viscosity for optimum wet film deposition.

Also, the incorporation of styrene-butadiene copolymer into the naturalrubber matrix will reduce the variations in strength results since thecopolymer reduces the initiation of failures caused by intrinsic filmflaws.

In general, the tear strength of the blend films increases with theconcentration of copolymer and peaks in the range of 10% to 15%concentration. This is consistent with the findings for the addition ofother reinforcing polymers to natural rubber. It is interesting to notethat the deviation of test results decreases as the concentration ofadditive increases. Again, this reflects more uniform films, since thedeviation of film thickness also decreases, as well as affirms the basicconcept that the tendency to tear is flaw dependent, and that theadditive makes the film more tear-resistant.

A value for % elongation at break is recorded although this will havequestionable validity as portions of the test specimen are subject todifferential strain effects and vary in tensile loading. However, sincethe specimens are all the same size, and mounted in the Instron testerin the same manner, the data can be used in comparative analysis. It isobserved in both experiments, and in general for polymeric additives,that with some small addition of copolymer, there is an increase in %elongation. This is likely attributable to the fact that since thecopolymer itself is an unsaturated, reactive copolymer, it will utilizesome of the available sulfur in intra-particle reaction, and thereforereduce the cross-link density in the bulk matrix, leading to higherextensibility. As the copolymer latex is increased in concentration, the% elongation passes through a maximum, and then begins to decreaseagain. This can be explained in that there will also be someinter-molecular cross-linking between the SB copolymer particles andnatural rubber. The particles will act as multinodal cross-link sites,and thus increase the overall cross-link density at higherconcentration. Also, there will be some structural dimensionallimitation as the natural rubber molecules are limited in their abilityto extend.

Tensile Strength

Tensile strength data were determined by two separate tests, i.e. usingdumbells as standard for rubber sheet testing, and by a ring test usedfor condom sections. The actual values of tensile strength in MPa areconsistent for both tests. However, the % elongation at break appears tobe marginally higher in the dumbell test. With the addition ofcopolymer, there is a steady increase in tensile strength up to 15%concentration of copolymer.

As observed before for tear, the % elongation at break in tensiletesting increases a small percentage with 5% addition, and then steadilydecreases with additive concentration. Again, the deviation of testresults is improved as the blend concentration increases.

Effect of Thermal Aging

For one set of films, the tear strength was measured on films that wereaged in an air convection oven for one week at 70° C. (This isequivalent to 64 months of exposure at room temperature.) The data ofTables 5 and 8 show that the tear strength is significantly reduced bythis aging process. For all of the films, i.e. natural rubber plus thefour blend compositions, the tear strength is reduced by about one-halffrom its initial value. There is also a loss of extensibility on airheat aging with % elongation being reduced by 25% for natural rubber andsomewhat less for blends as the concentration of SB copolymer increases.In general, the films of blends with 10% to 15% SB copolymer exhibittear strength after accelerated heat aging equivalent to that for unagednatural rubber film.

COMMERCIAL CONDOMS

One brand of commercial condoms was tested (with 25 specimens) for theirtear strength and tensile strength as a standard for current performanceand quality and the manufacturing process. The brand tested isnon-lubricated, manufactured from quality Natural Rubber Latex.

The data of Tables 9 and 10 summarize the test results. The tearstrength of commercial condoms is lower than that for films produced inthe laboratory. This is attributed, in part, to the fact that thecommercial condoms were about six months old when tested, and it isknown that strength properties decrease with time.

                  TABLE 1                                                         ______________________________________                                        COMPOUND FORMULATION (DRY BASIS)                                                          PHR                                                               COMPONENT     Control    Blends                                               ______________________________________                                        NR Latex      100        95    90    85  80                                   SB Latex      --          5    10    15  20                                   Potassium hydroxide                                                                         0.5                                                             Potassium laurate                                                                           0.5                                                             Sulfur        1.25                                                            ZDC           1.0                                                             Zinc oxide    1.0                                                             Antioxidant   1.0                                                             ______________________________________                                         The ingredients are described below, following Table 2.                  

                  TABLE 2                                                         ______________________________________                                        WET COMPOUND FORMULATIONS FOR 56% SOLIDS                                      % Solids      NR Masterbatch                                                                             20 phr SBR Blend                                   ______________________________________                                        NR Latex                                                                              62        4839     g     3870                                         SB latex                                                                              50        --             1200                                         Potassium                                                                             10        150            150                                          hydroxide                                                                     Potassium                                                                             20        75             75                                           laurate                                                                       Sulfur  62        60             60                                           ZDC     50        60             60                                           Zinc Oxide                                                                            50        60             60                                           Antioxidant                                                                           40        75             75                                           Water   --        321            90                                                             5640           5640                                         ______________________________________                                    

MATERIALS 1. Natural rubber latex

The Natural Rubber latex used is a Firestone Hartex 104 high-ammonianatural rubber latex, supplied by General Latex & Chemicals Ltd.Brampton, Ontario. Solids content is 62.0% w/w.

2. Styrene-butadiene latex

The SB latex was DOW SB 816, supplied by DOW Chemical Canada Inc. Thepolymer composition is 81% styrene: 19% butadiene with glass transitiontemperature of 45° C.

    ______________________________________                                        Other specification data are as follows:                                      ______________________________________                                        Solids, %                49-51                                                pH                       8.5-9.5                                              Styrene level            81%                                                  Particle size (Å)    1850-2550                                            Surface Tension (Dynes/cm)                                                                             46-60                                                Tg, °C.           45                                                   Alkali sensitivity       Low                                                  Brookfield viscosity (cps)                                                                             Below 150                                            Non-film forming at room temperature                                          ______________________________________                                    

3. Potassium hydroxide solution prepared as 10% w/w solution of BDHChemicals "assured" grade KOH (98% purity) in distilled water.

4. Potassium laurate solution is 20% w/w of potassium laurate (Pfaltz &Bauer Inc.) in distilled water.

5. The ZDC accelerator is a 50% w/w aqueous dispersion of zinc diethyldithiocarbamate prepared as ETHAZATE 50D from Uniroyal Chemicals Ltd.

6. Sulfur is a 60% w/w aqueous dispersion supplied by General Latex &Chemicals Ltd.

7. Zinc Oxide (ZnO) is a 40% w/w aqueous dispersion supplied by GeneralLatex & Chemicals Ltd.

8. The Antioxidant is a 40% w/w aqueous dispersion of Goodyear'sWingstay L prepared by General Latex and Chemicals Ltd.

                  TABLE 3                                                         ______________________________________                                        TEAR STRENGTH (EXAMPLE 1)                                                                         Elongation                                                                              Specimen                                                  Tear Strength                                                                           at break  Thickness                                                 (N/cm)    (%)       (mm)                                            ______________________________________                                        All Natural Rubber                                                                        21 Test specimens                                                 Mean        714         711       0.065                                       Std. Dev.   187          86       0.020                                       Min.        439         611       0.040                                       Max.        998         873       0.101                                       5% SB/NR Blend                                                                            25 Test specimens                                                 Mean        1034        826       0.059                                       Std. Dev.   121          49       0.016                                       Min.        805         705       0.038                                       Max.        1205        899       0.091                                       10% SB/NR Blend                                                                           24 Test specimens                                                 Mean        1166        745       0.047                                       Std. Dev.   111          32       0.009                                       Min.        947         693       0.033                                       Max.        1362        790       0.067                                       15% SB/NR Blend                                                                           24 Test specimens                                                 Mean        1223        698       0.052                                       Std. Dev.   122          31       0.014                                       Min.        957         635       0.033                                       Max.        1392        747       0.087                                       20% SB/NR Blend                                                                           19 Test specimens                                                 Mean        1206        618       0.047                                       Std. Dev.    78          26       0.011                                       Min.        1033        563       0.031                                       Max.        1358        666       0.066                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        TENSILE STRENGTH (EXAMPLE 1)                                                  Dumbell Test per ASTM D412                                                                            Elongation                                                       Tensile Strength MPa                                                                       at Break (%)                                          ______________________________________                                        All Natural Rubber                                                            Mean         24.6           982                                               Std. Dev.    7.2             85                                               Min.         13.2           867                                               Max.         33.4           1108                                              5% SB/NR Blend                                                                Mean         27.0           940                                               Std. Dev.    7.1             81                                               Min.         15.5           774                                               Max.         35.8           1018                                              10% SB/NR Blend                                                               Mean         26.9           856                                               Std. Dev.    3.4             82                                               Min.         20.2           732                                               Max.         31.3           957                                               15% SB/NR Blend                                                               Mean         26.4           786                                               Std. Dev.    2.8             57                                               Min.         22.2           695                                               Max.         30.2           849                                               20% SB/NR Blend                                                               Mean         22.9           730                                               Std. Dev.    0.7             73                                               Min.         22.0           645                                               Max.         23.7           824                                               ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        TEAR STRENGTH OF FILMS (EXAMPLE 1)                                            After Aging 7 Days at 70° C.                                                                    Elongation                                                      Tear Strength (N/CM)                                                                        at Break (%)                                         ______________________________________                                        Natural Rubber                                                                Mean         345             592                                              Std. Dev.    104              39                                              Range        170-438         524-640                                          5% SB/NR                                                                      Mean         461             626                                              Std. Dev.     89              29                                              Range        322-587         593-672                                          10% SB/NR                                                                     Mean         681             658                                              Std. Dev.    132              54                                              Range        508-915         589-763                                          15% SB/NR                                                                     Mean         792             653                                              Std. Dev.    161              96                                              Range         597-1033       523-772                                          20% SB/NR                                                                     Mean         620             506                                              Std. Dev.    143             125                                              Range        416-833         308-707                                          ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        TEAR STRENGTH (EXAMPLE 2)                                                             Tear Strength                                                                           Elongation                                                          (N/CM)    at Break %                                                                              Thickness (mm)                                    ______________________________________                                        Natural Rubber                                                                Mean      630         771       .086                                          Std. Dev. 104          74       .028                                          Range     370-803     590-898   .035-.154                                     5% SB/NR                                                                      Mean      909         831       .067                                          Std. Dev. 109          55       .028                                          Range     650-1129    696-951   .036-.163                                     10% SB/NR                                                                     Mean      976         817       .067                                          Std. Dev. 121          46       .024                                          Range     661-1295    682-925   .034-.121                                     15% SB/NR                                                                     Mean      1030        679       .060                                          Std. Dev.  99          37       .022                                          Range     722-1294    551-742   .029-.098                                     ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        TENSILE STRENGTH (EXAMPLE 2)                                                          Tensile Strength (MPa)                                                                     Elongation at Break (%)                                  ______________________________________                                        Natural Rubber                                                                Mean      23.8           823                                                  Std. Dev.  3.2            28                                                  Range     17.7-32.8      720-852                                              5% SB/NR                                                                      Mean      27.2           836                                                  Std. Dev.  3.2            21                                                  Range     18.7-34.8      788-866                                              10% SB/NR                                                                     Mean      25.7           780                                                  Std. Dev.  2.3            33                                                  Range     20.4-29.1      702-832                                              15% SB/NR                                                                     Mean      24.6           720                                                  Std. Dev.  2.1            20                                                  Range     20.9-30.5      671-750                                              ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        TEAR STRENGTH OF FILMS (EXAMPLE 2)                                            After Aging 7 Days at 70° C.                                                                    Elongation                                                      Tear Strength (N/CM)                                                                        at Break (%)                                         ______________________________________                                        Natural Rubber                                                                Mean         415             568                                              Std. Dev.     64              31                                              Range        318-493         515-598                                          5% SB/NR                                                                      Mean         567             569                                              Std. Dev.     74              33                                              Range        403-637         495-599                                          10% SB/NR                                                                     Mean         683             550                                              Std. Dev.    112              80                                              Range        520-814         433-665                                          15% SB/NR                                                                     Mean         621             438                                              Std. Dev.     61              41                                              Range        502-700         356-482                                          ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        TEAR STRENGTH OF COMMERCIAL CONDOMS                                           ASTM D624 DIE C                                                               Brand        Tear Strength (N/CM)                                                                          Range                                            ______________________________________                                        A-regular    529             369-598                                          A-lubricated 531             426-656                                          B-regular    474             408-572                                          B-lubricated 564             515-620                                          C-regular    605             438-809                                          C-lubricated 543             415-674                                          ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                         COMMERCIAL CONDOMS                                                           ______________________________________                                        Tensile Strength (MPa)                                                                            Elongation at Break (%)                                   ______________________________________                                        Mean    23.4            788                                                   Std. Dev.                                                                              5.2             43                                                   Range   11.5-33.9       677-856                                               ______________________________________                                        Tear Strength (N/CM)                                                                              Elongation at break (%)                                   ______________________________________                                        Mean    492             567                                                   Std. Dev.                                                                             185              82                                                   Range   309-981         445-770                                               ______________________________________                                    

What is claimed is:
 1. A prophylactic condom comprising a natural rubberfilm comprising cured natural rubber and a styrene-butadiene copolymer,wherein said copolymer contains greater than 50 weight percentpolymerized styrene, the remainder being polymerized butadiene, andwherein said copolymer is present in an amount within the range of fromabout 1 to 25 parts, by weight, per 100 parts by weight of naturalrubber.
 2. The prophylactic condom of claim 1 wherein saidstyrene-butadiene copolymer contains from about 75 to about 85 weightpercent polymerized styrene.
 3. The prophylactic condom of claim 1wherein the styrene-butadiene copolymer is present in an amount of fromabout 10 to about 20 parts, by weight, per 100 parts by weight ofnatural rubber.
 4. The prophylactic condom of claim 2 wherein thestyrene-butadiene copolymer is present in an amount of from about 10 toabout 20 parts, by weight, per 100 parts by weight of natural rubber.